1. Field of the Invention
This invention relates to telephone subscriber loop length detection and, more particularly, to the automatic detection of the resistance of telephone subscriber loops.
2. Description of the Prior Art
In order to provide telephone service on very long subscriber loops, it is often necessary to provide voice frequency gain and supervisory signal detectors at the central office end of the subscriber loop. Such specialized facilities are called "range extenders with gain" (REGs). One such range extender is shown in K. E. Stiefel U.S. Pat. No. 4,056,688, granted Nov. 1, 1977. The need for range extension of any particular subscriber loop is dependent upon the resistance of that loop. Short loops, for example, do not require range extension while very long loops cannot be serviced entirely from the central office end and usually require remote amplification units. A range extender which is automatically adaptable to any length of line is particularly desirable since the loop parameters need not be investigated for each loop prior to installation. One such automatically adjusting range extender is disclosed in the aforementioned patent of the present applicant.
In order to provide automatic adjustment to the length of the connected subscriber loop, it is customary to detect the level of current flow in the loop since such current flow is an indirect measure of the resistance, and hence the length, of the subscriber loop. In the presence of changing central office voltages or voltage reversals at the central office, such current detectors may become inoperative or may indicate an erroneous length of the loop. Providing separate current detectors for each polarity of central office voltage is expensive and does not guarantee accurate detection of subscriber loop length.
Other problems with prior art loop current detectors include sensitivity to power delivered from the loop by wire crosses. Moreover, such current detectors are also used to indicate the off-hook and dial pulsing conditions. During dial pulsing, the capacitance of the connected subscriber loop modifies the waveshape of the dial pulses considerably and renders the detection of dial pulse transitions difficult. Similarly, the detection of a loop current in the presence of high magnitude alternating current ringing signals presents a serious problem for such current detectors.